In high-speed machines, such as, for example, in high-speed fluid-flow machines, the connection between the impellers and the shaft must satisfy stringent requirements. These connections must be absolutely rotationally fixed at high speeds, despite the high forces which occur, they must be able to transmit high torques and have a long service life, but must nonetheless often be releasable for maintenance purposes.
Various possible ways of realizing such connections have been proposed. Thus, the impeller, for example, may be provided with a continuous central bore and the shaft may be provided with a tapped hole, so that the impeller can then be screwed on with a clamping screw. Or the impeller, on its side facing the shaft, is provided with a blind hole having an internal thread, and a threaded stem having a matching external thread is provided on the shaft end, so that the impeller can be screwed directly onto the shaft. Since the impellers are often produced from a material different from that of the shaft, problems repeatedly occur with these screwed connections on account of different coefficients of expansion of the materials, a factor which may lead to rotor unbalance during operation. The softer material may also sustain damage during repeated release and tightening in the region of the thread. Furthermore, “fretting” of the threads may occur. Thread furrows which are made directly in the hub of the impeller or in the shaft may lead to unfavorable notch effects in the components, a factor which may lead to the fracture of the hub in the case of threads in the hub. This is a very big problem in turbochargers having ram induction, the impellers of which are subjected to considerable torque fluctuations.
In order to avoid the problems caused by the threads, it has been proposed in the case of impellers screwed directly to the shaft to arrange a bush between the shaft and the impeller. Such a connection is known, for example, from Napier EGT 297 from “Diesel and Gas Turbine Catalog 1998, Volume 63; Page 686”. In this example, the compressor wheel of a turbocharger is provided on the shaft side with a hub extension which has a central blind hole. A bush is frictionally inserted into this blind hole. This bush has an axially extending bush part and, on the shaft side, an annular bush flange which projects radially outward therefrom and has approximately the same outside diameter as the end face of the hub extension. The bush part is dimensioned in such a way that it extends just up to the base of the blind hole when the bush flange is brought to bear against the end face of the hub extension. Two cylindrical recesses lying axially one behind the other are provided in the bush part, the impeller-side cylindrical recess having a smaller diameter than the shaft-side cylindrical recess and being provided with an internal thread. Formed between the two recesses is a shoulder which, in interaction with the shoulder of the shaft journal designed to be diametrically opposed to the inside of the bush, serves for centering the compressor wheel relative to the shaft. During assembly, first of all the bush is inserted frictionally into the central recess of the compressor wheel and then the compressor wheel together with the bush is screwed onto the shaft journal of the turbocharger shaft. The compressor wheel itself has no thread and is thus freed of the notch stresses. In addition, given appropriate selection of the bush material, the problem of the different coefficients of expansion and of the fretting and also of the damage to the softer thread during repeated release and tightening can be reduced or completely removed. In this design, the blind hole and end face of the hub extension, the bush together with flange, shoulder and thread, and also the shaft journal together with thread and shoulder must be worked very accurately, which is complicated and expensive.
In addition, this design leads to large leakage flows. Nonetheless, such a connection cannot be used for connections in which very high torques or high torque fluctuations have to be transmitted between impeller and shaft, as is the case, for example, in turbochargers having ram induction, since the torque transmission is inadequate.